National Aquatic Resource Surveys

National Lakes Assessment

The National Lakes Assessment (NLA) is a statistical survey of the condition of our nation's lakes, ponds, and reservoirs. It is designed to provide information on the extent of lakes that support healthy biological condition and recreation, estimate how widespread major stressors are that impact lake quality, and provide insight into whether lakes nationwide are getting cleaner.

Healthy lakes enhance our quality of life. We use lakes for drinking water, energy production, food, and recreation. Fish, birds, and other wildlife rely on them for habitat and survival. (Photo: Hilary Snook, EPA)

Conditions in our nation’s lakes, ponds and reservoirs are reported in the EPA’s National Lakes Assessment 2012 (NLA), one in a series of National Aquatic Resource Surveys designed to provide nationally-consistent and representative information on the condition of the nation’s waters.

Background: In the summer of 2012, 89 National Lake Assessment field crews sampled 1,038 U.S. lakes using consistent methods and following standardized quality assurance protocols. The surveyed lakes were identified using stratified random sampling and are representative of those found across the country, from small ponds and prairie potholes to large lakes and reservoirs.

Nutrient Pollution: The NLA finds that high levels of the nutrients phosphorus and nitrogen are widespread in lakes. Nutrient pollution can contribute to unsightly algal blooms, too little oxygen, potential harm to aquatic life and human health, and impaired recreation. Human activities such as poorly managed agriculture or suburbanization of watersheds can result in nutrient pollution.

Findings for Nutrients: The NLA finds that 40% of lakes are in most disturbed condition for phosphorus and 35% are most disturbed for nitrogen. Excess levels of these nutrients in lakes are associated with increased risk of degraded biological communities.

Trophic Condition: Eutrophication – an increase in nutrients that also produces excessive plant growth -- is a slow and natural part of lake aging. Human influences can accelerate eutrophication and its effects, such as nuisance algae, murky water, low levels of oxygen, odor, and fish kills.

Findings for Trophic Condition: Based on measures of chlorophyll-a used to estimate trophic status in the NLA, more than half of lakes are eutrophic or hypereutrophic (most disturbed).

Biological Condition: The NLA 2012 finds that 31% of lakes have degraded communities of bottom-dwelling insects and small aquatic animals like snails and crayfish. In 21% of lakes, communities of zooplankton (tiny animals in the water column) are in degraded condition.

Physical Habitat: The condition of lakeshore habitats provides important information on lake health. Healthy lakeshore habitat slows pollution runoff and provides varied and complex ecological niches for aquatic life. More than one quarter of U.S. lakes are in most disturbed condition for vegetation along the lakeshore and at the land-water interface. (Photo: Hilary Snook, EPA)

Recreational Condition – Algal Toxins: Algae and cyanobacteria are a natural part of freshwater ecosystems. However, some algae blooms, powered by high levels of nutrients and warm temperatures, can be harmful to people and animals. An algal toxin, microcystin, is detected in 39% of lakes, but concentrations reach levels of concern in less than 1% of lakes. (Photo: Ellen Dickey, DNREC)

Recreational Condition – Atrazine and Mercury: The NLA looked at other indicators of recreational condition in lakes: atrazine, a widely used agricultural herbicide, and mercury in sediment. Atrazine was detected in 30% of lakes although at levels of concern in less than 1% of lakes; 40% of lakes are in most disturbed condition for methylmercury in sediment. (Photo: Hilary Snook, EPA)

Change in Lake Condition: A comparison of the results of the NLA 2012 to those of an earlier study in 2007 show little change in lake condition between surveys. However, one area of change is in the detection of microcystin, which increased by nearly 10%. Another difference emerged through additional in-depth analyses of nutrient data. While we did not observe changes in the nutrient condition categories, analysts found a dramatic 18.2% decline in the percentage of oligotrophic lakes (<10 µg/L of total phosphorus). (Photo: Hilary Snook, EPA)